A planographic printing plate

ABSTRACT

A planographic printing plate formed of a base sheet having the combination of an outer lithographic surface coating and a base or barrier coating immediately underlying the lithographic surface coating in which the surface coating is formulated of polyvinyl alcohol and a filler and containing a phosphate of ammonia or a monovalent metal and in which the underlying base coating is formulated of polyvinyl alcohol, filler and insolubilizing agent in the form of a melamine formaldehyde resin and containing water soluble polyvalent metal salts.

United States Patent William H. Page;

Henry P. Williams, both of Appleton, Wis. 792,485

Jan. 21, 1969 Nov. 23, l 97 1 Appleton Coated Paper Company Appleton, Wis.

inventors Appl. No. Filed Patented Assignee PLANOGRAPl-IIC PRINTING PLATE 20 Claims, 1 Drawing Fig.

US. Cl 117/76 R, 96/33, 117/76 P, 117/84, 117/86, 101/460, 101/462, 101/473 Int. Cl 841m 1/06 Field ofSearch 117/76 P, 76 F, 75, 84, 86; 101/460, 461, 462, 453, 473;

L/rHoamPH/c swam CECHAT/NG 14- PoI jgylA/cqkol C 01 a1 .Sflzca Sodium Aczd Pyrophnsphaz c Primary Examiner-William D. Martin Assistant Examiner-Ralph l'lusack AttorneyMcDougall, Hersh, Scott & Ladd ABSTRACT: A planographic printing plate formed of a base sheet having the combination of an outer lithographic surface coating and a base or barrier coating immediately underlying the lithographic surface coating in which the surface coating is formulated of polyvinyl alcohol and a filler and containing a phosphate of ammonia or a monovalent metal and in which the underlying base coating is formulated of polyvinyl alcohol,

filler and insolubilizing agent in the form of a melamine formaldehyde resin and containing water soluble polyvalent metal salts.

(PA PER BASESHEET l0 BASECOA TING 72 P 1 irgyl Aloe/10L M ammefqr ma e/zyde. a 265m Resin elamirzc I 0231mm Chlorzag Zinc Acetate PATENTEDN V 23 3,622.37 3

LITIZOGRAPHICASUfJZ4 czcoAr/A/a 14 0! um 7 [c0 0 00 pi a] Silica Sodium 401d Pyrophosphal e INVENTORS ZUilIiam N Page,

by I New Plllz'lliams (Z1495 A PLANOGRAPHIC PRINTING PLATE This invention relates to planographic printing plates and more particularly to a coated paper lithographic plate which may be used as a direct image plate.

The lithographic surface of a coated planographic printing plate is one in which the surface of the coating is water insoluble, water receptive, ink repellent and lithographic such that, when the surface imaged with a greasy imaging material is first wet with an aqueous repellent and then with a greasy ink, the aqueous repellent will wet out the nonimaged hydrophilic surface portion of the plate leaving the oeophilic, ink receptive, water repellent imaged portion free to accept ink for transfer to copy sheets, directly or through an offset blanket, to produce multiple copies.

To the present, such lithographic coatings have been formulated of a hydrophilic adhesive or colloid, such as starch, casein, polyvinyl alcohol, alginates, carboxymethylcellulose, polyacrylates and the like insolubilized with a suitable acid or salt and pigmented with a finely divided inorganic material such as kaolin clay, blanc fixe, colloidal silica, zinc oxide, titanium dioxide and the like.

When the lithographic coating is applied to a paper base sheet in the preparation of a coated paper lithographic master, the surface of the paper is usually first coated with a formulation designed to be a seal or barrier to water in the liquid phase, which also imparts wet strength to the paper and provides a suitable adhesive base for the lithographic surface coating. The more recent trend is towards the reduction of the number of coatings applied to the paper base sheet with the ultimate goal of preparing a planographic printing plate which requires the application of but a single coating composition in plate manufacture.

it is an object of this invention to produce and to provide a method for producing a coated paper lithographic plate which conforms to the general trend in that it limits the number of coatings to two on the functional side and relies upon the combination of a lithographic surface coating and a base coating separately applied to the paper base sheet to provide a coated paper lithographic plate which is characterized by greater versatility in use, which has good shelf life, which embodies an excellent balance between the hydrophobic and hydrophilic properties of the plate and which enables a greasy image to be strongly bonded to the surface of the plate for the production of a large number of copies of good quality, which can be used as a direct image plate to produce a large number of copies with a wide variety of imaging and wetout materials thereby to provide a universal plate that is compatible with the use of materials of others, and which can be presensitized with lightsensitive materials to form an ink receptive image upon exposure to light, and it is an object of this invention to produce and to provide a method for producing a planographic printing plate of the type described.

These and other objects and advantages of this invention will hereinafter appear and for purposes of illustration, but not of limitation, an embodiment of the invention is shown in the accompanying drawing which is a perspective view, partially in section, of a planographic printing plate embodying the features of this invention.

The planographic printing plate of this invention is based upon a system of dual coatings including a lithographic surface coating containing an adhesive, such as polyvinyl alcohol or hydroxyethylated starch, as the sole hydrophilic colloid adhesive with suitable pigments or fillers and a base coat immediately underlying the lithographic surface coating formulated of polyvinyl alcohol as an essential component of the adhesive with suitable pigments or fillers and multivalent metal salts which operate to catalyse the insolubilization reaction of the lithographic surface coating from the underside without the necessity of having the acidic catalysing salts in or on the lithographic surface itself.

The novelty of such construction lies in the fact that it is in exact reverse to that of currently accepted practice in the manufacture of paper offset masters, wherein multivalent salts are usually contained either in situ in the topcoat or are applied in an extra coating operation from a specially formulated solution of salts or salts plus adhesive.

Another area of novelty is the use of phosphates of ammonia or monovalent metals in the topcoat expressly to furnish anions to react with the multivalent metal ions in the base or barrier coat to form an efiicient pore plugging precipitate in the interstices of and at the interface of the barrier coat and the topcoat. I

Moving the source of the multivalent metal salts from the plate surface to a position below the functional surface also enables us to maintain the surface pH at a level favorable to the drying of oleoresinous or varnish based ink without losing the functional advantages which multivalent metal salts are known to contribute to pigmented lithographic coatings.

In this context, it is interesting to theorize on the mechanism by which these salts contribute to lithographic efficiency. US. Pat. No. 2,534,650 l 2-19-50) teaches that "the water soluble and water stable bivalent metal salt tends to enter and plug up the interstices, that is, the capillary pores of spaces, in or among the finely divided mineral particles and colloid embodied in the coating, particularly when a wetting out solution is used which reacts with the bivalent metal salt to produce a bulky precipitate. One of the ingredients of most wetting out solutions is a phosphate of a monovalent metal, usually sodium dihydrogen phosphate. It is chemically sound reasoning to expect that this ingredient of wetting out solution reacts with zinc ions and form a fiocculent precipitate as described in the above mentioned patent.

In our reverse construction, we have incorporated aluminum and zinc salts or other polyvalent metal salts in the barrier or undercoat and a soluble phosphate in the topcoat. The chemical logic of this move will be immediately evident when one considers that a fiocculent mixed precipitate of both aluminum and zinc salts forms near the interface of the undercoat and the topcoat when the topcoat comes into contact with the undercoat in the topcoating operation. This fiocculent material is presumed to fill in more completely pores in the barrier coat, increasing its efi'rcency and its ability to retard the flow of excess free Al-l-l-l- Cl, Zn -lland/or the polyvalent metal ions or anions that move upward through the face coat, thus preventing the face coating from becoming sensitive" to fingerprinting or from becoming more acidic than is consistent with the normal drying of greasy inks, such as are used for the preprinting of reproducing images on plates which are going to be used for making business forms or bank checks.

Amongst the unique principles of our plate paper manufacturing system, are (l) the reverse construction which places the sources of the multivalent metal salts in the under or barrier coat, (2) the absence of multivalent metal salts in the lithographic topcoat formulation and (3) the use of a selected water soluble phosphate in the topcoat, which reacts with a portion of the multivalent metal salts in the undercoat and which may obviously be adapted for use with any compatible hydrophilic colloidal adhesive. For instance, hydroxyethylated starch may be substituted for polyvinyl alcohol in formula A" hereinafter set forth.

While we have found in our work that polyphosphates, like sodium acid pyrophosphate and sodium hexametaphosphate, give the best results in lithographic topcoat formulas, any soluble phosphate which will furnish anions to react with the aluminum and zinc ions to form a precipitate in the pores of and at the interface of the barrier coat and the lithographic topcoat would enhance the lithographic properties of the system and would be within the scope of the invention. Thus, any of the soluble sodium, ammonium, potassium, etc. phosphates could be used, such as monosodium phosphate, disodium monohydrogen phosphate, tetrasodium pyrophosphate, sodium acid pyrophosphate, sodium tripolyphosphate and sodium hexameta phosphate.

Any colloidal adhesive can be employed that is able to hold the multivalent metal salts in solution without coagulation until the coating has been applied to the paper and the water is removed in the drying process. Likewise, the topcoat adhesive should be one that is compatible with these same salts in order for it to form a firm bond with the undercoat. Sodium alginate would be incapable of use as an adhesive since it is coagulable with metal ions and would tend to form a poor bond between undercoat and topcoat due to the formulation of a weak interfacial layer of zinc and/or aluminum alginate.

The pigmentation of the topcoat is chosen to make the surface as universally receptive as possible to all common imaging media. In the practice of this invention, it is preferred to make use of aluminum modified silica as a filler or pigment in the top coat to enhance the bond of greasy based images deposited on fabric typewriter ribbons or from ball point pens. It is preferred to make use of a ratio of Kaolin clay to silica to provide for porosity and keying action to hold waxy images deposited from carbon paper or plastic ribbons. It is desirable to include silica amongst the pigments of the top coat to provide a slight abrading action to accept and hold images deposited from reproducing pencils. Others of the conventional fillers or pigments can be employed.

Taken as a whole, the unique construction of this complimentary system of coatings makes the resulting product the first truly all purpose plate, as the following listing of end uses will attest.

I. General direct image work-the plate will accept and tenaciously hold images from any imaging device in common use-for the reasons outlined directly above, plus the fact that image hold on areas abraded and polished by an eraser is insured by the fact that the act of erasing exposes a somewhat more chemically image receptive surface than the original surface. The chemical assistance which the salts are known to contribute to image hold thus compensates for the partial loss of keying and porosity due to the abrading and polishing action of the erasing process.

2. Xerox transfer workthe thermoset undercoat plus a top and back coat designed to be free of hygroscopically unstable materials makes for absence of cockle and generally high quality Xerox performance.

3. Repro preprint work-since the plate surface is essentially neutral and free of chemicals known to have an inhibiting effect on the drying of oil or varnish based printing inks, it is remarkably well adapted for this kind of work.

4. VerifaxEktalith Briefly described, in the preferred practice of this invention, the lithographic surface coating is formulated of polyvinyl alcohol as the sole hydrophilic colloid and one or more fillers selected from the group consisting of kaolin clay and colloidal silica but which may instead be formulated with other conventional pigments or fillers, such as titanium dioxide, zinc oxide, and the like. it will be noted that the described lithographic surface coating does not contain any salts of multivalent metals to modify the hydrophilic lypophilic balance. Similarly, wash coating of the lithographic surface with polyvalent metal salts in solution for insolubilization or modification of the hydrophilic lypophilic balance is not employed thereby to avoid overcoating of the lithographic surface with an excess of water soluble metal salts which might detract from the ability strongly to anchor the image onto the lithographic surface and/or which might otherwise react nonuniformly with the surface coating materials in the reaction for insolubilization and which are known to exert a negative influence on the dry ing of inks used for preprinting plates used in the forms or bank check business.

The base coat immediately underlying the lithographic surface coating is formulated of an adhesive system which includes polyvinyl alcohol as an essential component and which in the preferred practice of this invention, is formulated to include melamine formaldehyde and melamine. Suitable pigments. such as clay or other of the fillers previously described, are formulated into the base coat with water soluble polyvalent metal salt which is believed to operate from the base coat to provide for controlled image retention by the lithographic surface coating.

While polyvinyl alcohol is the preferred adhesive for this invention, it will be readily appreciated that the reverse construction of this invention is essentially independent of the adhesive system and that any colloidal hydrophilic adhesive which is not irreversibly coagulate by the addition of multivalent salts or is not irreversibly coagulated when brought into contact with surfaces containing multivalent metal salts will work and may be substituted for the polyvinyl alcohol in the surface coating and in the base coat, such as represented in the examples hereinafter set forth. Hydroxyethylated starch is an example of an adhesive of the general type specified by the invention.

Other minor ingredients may be included both in the lithographic surface coating and in the underlying base coat, such as surface active agents, foamicides, fungicides and the like, which have to do more with the stabilization of the coating compositions and the preservation of the coatings rather than the lithographic characteristics of the combination of lithographic surface coating and base coating.

The invention will now be described in greater detail with reference to the following example:

satin) Min. & Chem. 1.8 Melamine formaldehyde resin, 40% by weight solids (parez resin #613- American Cyanamid Company). 1.0 Defoamer (equal parts tributyl phosphate and butanol).

0.5 40% solids (Foamaside #581 Colloids Inc.).

123.7 Water.

1.4 Aluminum chloride solution, 50%

solids.

6.5 Zinc acetate solution, 25% solids.

2Q] Water.

In the above formulation, the Cypel emulsion comprises a mixture of approximately 63 percent by weight ethyl acrylate, 35 percent by weight acrylonitrile and 2 percent by weight methacrylic acid and is marketed by the American Cyanimid Company. The polyvinyl alcohol is first dispersed in the first increment of water and dissolved by heating to about C. Thereafter all of the ingredients with the exception of the metal salts and the second increment of water are combined in a suitable blender and then the salts and the remainder of water added to fonn the slip.

Application is made to a suitable base paper 10 of about 50 pounds per ream of 500 sheets of 25 inches X 38 inches. Application is made by a suitable coating technique, such as by roller coating or flow coating in a dry coating weight of about 8 pounds per 3300 square feet or surface area. Excess coating composition can be removed by means of an air knife nd the coating is dried by passage through an air drying oven at a temperature of about to 200 C. for l to 30 minutes exposure.

EXAMPLE 2 Lithographic surface coating composition Pounds:

5. 4 Polyvinyl alcohol (Gelvatol 39l) Monsanto. 36. 0 Clay slurry, 68% solids (Ultra White 90) Minerals and Chemicals Philipp Corporation.

43. 2 Colloidal silica, 30% by weight solids (Ludox AMMonsanto Chemical Company).

3. 3- Melamine formaldehyde resin emulsion, 40% solids (Resloom M-80Monsan to Chemical 75% solids in water (dioctyl ester of sodium sulfosuccinate) American Cyanamid.

8. 9 Sodium acid pyrophosphate, 7% solution in water. 102. 4 Water.

Again, the polyvinyl alcohol is first dispersed in cold water and then heated to about 90 C. to effect solution and the remainder of the ingredients are blended to form the coating mix.

The lithographic surface coating composition is applied as, an overcoat 14 onto the dried base coating 12 in an amount to provide a dry coating weight of 4 pounds per 3000 square feet of surface area. Application can be made by roller coater or by flow coating followed by an air knife for the removal of excess coating composition and smoothing rolls to smooth the coating. The lithographic surface coating 14 is dried by passage through an air drying oven at a temperature of about 100 to 250 C. for about to 15 minutes.

The polyvinyl alcohol component present both in the base coat and the lithographic surface coating formulations may contain a small amount of polyvinyl acetate, such as between 2-10 percent by weight polyvinyl acetate, such as marketed by Shawinigan Chemicals, lnc. under the trade name Gelvatol 3-91." The polyvinyl alcohol can be formulated in the base coating composition in an amount within the range of 7-35 percent by weight and preferably in an amount within the range of 15-29 percent by weight, calculated as solid polyvinyl alcohol in the coating composition, and it may be formulated in the lithographic surface coating composition in an amount within the range of 4-20 percent by weight and preferably in an amount within the range of 8-16 percent by weight solid polyvinyl alcohol in the surface coating composition.

The melamine formaldehyde resin in the base coat can be represented by the resin marketed by Monsanto Chemical Company under the trade name Resloom M-80 or by American Cyanamid under the trade name Parez 613." The melamine formaldehyde resin can be replaced by other amide aldehyde resins, such as dimethylol urea and urea formaldehyde resin. In the base coating composition, the melamine formaldehyde or equivalent amide aldehyde resin can be employed in an amount within the range of 0.4-4 percent by weight solids and preferably in an amount within the range of 1.3-6 percent by weight solids. When employed in the lithographic coating composition, the component identified as the melamine formaldehyde resin can be employed in an amount within the range of 0.5-5 percent by weight and preferably in an amount within the range of 2-4 percent by weight calculated on the dry solids basis of melamine formaldehyde resin in the coating composition.

The ratio of ethyl acrylate, acrylonitrile and methacrylic acid in the Cypel resin in the base coating composition may be varied within the range of 55-85 parts by weight ethyl acrylate, -45 parts by weight acrylonitrile and 0.5-4 parts by weight methacrylic acid. The Cypel resin component, which identifies the aforementioned resinous system, can be employed in the base coating composition in an amount within the range of 3-15 percent by weight and preferably 5-10 percent by weight when calculated on the basis of its dry solids.

Instead of clay in the base coat and/or the lithographic top coat, use can be made of other suitable pigments such as Blanc Fixe, titanium dioxide, colloidal silica and the like fillers. The amount of pigment or filler can range from 1-4 parts by weight pigment or filler in the base coat to 1 part by weight binder or, when calculated on the basis of the solid pigment or filler in the coating composition, 40-80 percent by weight pigment and preferably 50-70 percent by weight pigment in the base coating composition In the top coating composition, the

amount of pigment may be employed within the range of one to eight times the amount of polyvinyl alcohol and preferably six to seven times the amount of polyvinyl alcohol, or in an amount within the range of 75-90 percent by weight and preferably 81-86 percent by weight solids in the lithographic coating composition.

The colloidal silica component in the lithographic surface coating composition is preferably selected of a colloidal silica in which part of the silicon has been replaced with aluminum, such as marketed by 'E. 1. duPont de Nemours & Company under the trade name Ludox AM having 30 percent solids in aqueous medium. Other colloidal silicas marketed by Monsanto, such as under the trade name Syton" or by E. l. du Pont under the trade name Ludox HS or by G. 1. Cabot Corporation under the name Cab-0-Sil can be used. The colloidal silica component employed in the color coating or top coat may vary within the range of 8-40 percent by weight and preferably 10-35 percent by weight when calculated on the basis of the solid colloidal silica in the coating composition.

The defoamers and surface active agents represent additives which function to stabilize the coating composition and their application to the paper base sheet. It will be understood that other defoamers, surface active agents and wetting agents conventionally employed in the coating art can be substituted in variable amounts without departing from the spirit of the invention.

The aluminum chloride and zinc acetate in the base coating composition can be replaced with other water soluble polyvalent metal salts such as the chlorides, nitrates, formates, acetates and propionates of other bivalent metals, such as zinc, calcium, nickel, copper, cobalt, barium, strontium, and the like, or trivalent metals such as aluminum. it is preferred to employ a combination of the bivalent and trivalent metal salts, such as zinc chloride or zinc acetate and aluminum chloride with the materials present in the ratio of 1-4 parts by weight of the bivalent metal salt to one part by weight of the trivalent metal salt. The base coat composition can be formulated with the polyvalent metal salts present in an amount within the range of 2-15 percent by weight and preferably 5-8 percent by weight of the coating composition.

The following are further examples of compositions embodying the features of this invention for use in the preparation of lithographic printing plates:

EXAMPLE 3 Long run plate-Lithographio surface coating composition Pounds:

104 Water. 5.5 Polyvinyl alcohol. 31.4 Colloidal silica, 30% by weight solids (Ludox AM). 9.4 Colloidal silica, 40% by weight solids (Ludox HS). 36.5 Clay slurry, 68% solids. 3.4 Melamine formaldehyde resin, 40%

solids (Resloom M-). 1.1 Melamine. 0.6 Defoamer. 0.2 W Aerosol OT wetting agent. 9.0 Sodium acid pyrophosphate, 7% solution in water.

EXAMPLE 4 Long run plate Base coating composition: pounds 123.7 water 1.5 melamine recrystallized 7.7 polyvinyl alcohol 29.4 Ultra While clay slurry, 68% solids 5.8 Cypel resin, 44% solids melamine formaldehyde resin. 40% solids defoamer aluminum trichloride, 50% in water The composition is the same as that of example except that cobalt acetate is substituted for the equivalent amount of zinc acetate.

EXAMPLE 7 Base coat composition: The formulation is the same as that of example 5 except that nickel acetate is substituted for the zinc acetate.

EXAMPLE 8 The fonnulation is the same as the base coat composition of example I except that the Cypel resinous component is omitted.

The combination of melamine, Cypel resin, polyvinyl alcohol and melamine formaldehyde resin in the base coat composition provides a self-curing system that operates to bond the mineral pigments to the underlying paper base sheet to provide a base coat having controlled hydrophilic characteristics and which operates as a water barrier. The polyvalent metal salts are present in the base coat in an amount sufiicient to maintain the desired balance between hydrophilic and hydrophobic characteristics while supplying the top coat with a continuous source of polyvalent metal salts as required to maintain the desired balance in the lithographic surface coating notwithstanding the amounts removed in use of the plate by the pre-etch and fountain solutions with which the plate surface is continuously wet during the production of copy. Thus the base coat provides a unique function as a well which is protected by the covering top coat from the processing solution but from which withdrawal can be made for optimum lithographic characteristics in the lithographic surface coating. This represents a complete departure from the concepts of plate production and utilization as heretofore practiced.

An advantage of this reverse construction resides in the simplification for control of insolubilization of the coatings while maintaining optimum lithographic properties. In the conventional constructions wherein the polyvalent metal salts are applied as a wash coat over the lithographic surface coating, the amount of salts must be carefully controlled, otherwise excess salts must be removed as by brushing or burnishing. Even then, wide variation in salt concentration occurs and a metal salt layer invariably remains as an overcoating and interferes with the ability properly to anchor the grease image.

A further advantage of the combination of coatings of the type representing the features of this invention resides in the ability to make corrections by erasure or image removal while still maintaining surface characteristics sufficient strongly to anchor the redeposited image. in prior processes which make use of a wash coating of polyvalent metal salts, image removal by erasure operates not only to remove the image but also some of the prime ingredients essential to the lithographic coating thereby to render the surface unsuitable for re-imaging to make corrections and the like.

While not as effective or equivalent to the system wherein melamine formaldehyde resin is present as a component in the lithographic surface coating or wherein the polyvalent metal salts are present in the base coat, a workable lithographic plate can be fabricated embodying the reverse arrangement when melamine is deleted as a component in the lithographic surface coating or when the polyvalent metal salts are deleted as a component of the base coat. In the latter instance, reliance is had on he amide aldehyde component of the base coat to contribute the desired insolubilizing effect to the lithographic surface coating immediately overlying the base coat.

The following are representative of the above modifications:

As in the present formulation of example 1, the polyvinyl alcohol is first dissolved by dispersion in cold water and heating to elevated temperature and then the rest of the ingredients are added. Application is made to base paper in coating weights of about l0 pounds per 3000 square feet of surface area and the coating is dried at a temperature of C. for 10 minutes.

in the above formulation of the base coat suitable for use with the top coat of examples 2, 3 and 4, the polyvalent metal salts are absent and reliance is had on the melamine formaldehyde for maintaining the desired balance.

EXAMPLE 10 The formulation is the same as that of example 2 except that the melamine-formaldehyde component is omitted. in this formulation reliance is had on the polyvalent metal salts in the base coat of examples 1, 5, 6, 7 or 8 to provide the desired balance for insolubilization of the adhesive and formation of the pore plugging agents;

It will be apparent from the foregoing that we have provided a new concept in lithographic plate production whereby a lithographic printing plate is produced which is continuously controlled for optimum lithographic characteristics for retention of the image and for the production ofa large number of copies of good quality and in which the plate is capable of use with a wide variety of materials and corrections without interfering with the ability of the plate to produce copy.

it will be understood that changes may be made in the details of formulation and operation without departing from the spirit of the invention, especially as defined in the following claims.

We claim:

1. A planographic printing plate comprising a base sheet and the combination on the surface of the base sheet of an outermost lithographic surface coating and a base coating immediately underlying the surface coating in which the surface coating is formulated of a hydrophilic colloid adhesive which is not coagulated from the coating solution by polyvalent metal salts and which is substantially free of polyvalent metal salts, a soluble phosphate which reacts with polyvalent metal salt to form a bulky precipitate and a pigment, and in which the underlying base coat is formulated of a hydrophilic colloid adhesive which is not coagulated from the coating solution upon contact with a polyvalent metal salt, and water soluble polyvalent metal salts.

2. A planographic printing plate as claimed in claim 1 in which the hydrophilic colloid adhesive of the lithographic surface coating is selected from the group consisting of polyvinyl alcohol and hydroxyethylated starch.

3. A planographic printing plate as claimed in claim 1 in which the hydrophilic colloid adhesive in the base coat is selected from the group consisting of polyvinyl alcohol and hydroxyethylated starch.

4. A planographic printing plate as claimed in claim 1 in which the pigment the lithographic surface comprises colloidal silica containing substituted aluminum.

5. A planographic printing plate as claimed in claim 1 in which the soluble phosphate is selected from the group consisting of monosodium phosphate, disodium monohydrogen phosphate, tetrasodium pyrophosphate, sodium acid pyrophosphate, sodium tripolyphosphate, sodium hexametaphosphate and the corresponding phosphates in which ammonium or potassium are substituted for sodium.

6. A planographic printing plate as claimed in claim 1 in which the adhesive of the base coat includes an amide aldehyde resin.

7. A planographic printing plate as claimed in claim I in which the adhesive of the base coat includes an amide aldehyde resin and an ethyl acrylate-acrylonitrile-methacrylic acid resin.

8. A planographic printing plate comprising a base sheet and the combination on the surface of the base sheet of an outermost lithographic surface coating and a base coating immediately underlying the surface coating, in which the surface coating is formed of the dried residue of an aqueous composition the solids of which consist essentially of polyvinyl alcohol, a soluble phosphate and a filler or pigment without polyvalent metal salts and in which the base coat is formed of the dried residue of an aqueous composition in which the solids consist essentially of polyvinyl alcohol, a melamine formaldehyde resin, an ethyl acrylate-acrylonitrile-methacrylic acid resin,

filler or pigment and water soluble polyvalent metal salts.

9. A planographic printing plate as claimed in claim 8 in which the filler or pigment comprises the combination of clay and a colloidal silica.

10. A planographic printing plate as claimed in claim 8 in which the filler or pigment includes a colloidal silica in which at least a part of the silicon is replaced withalu nin m 11. A planographic printing plate as claimed in claim 8 in which the filler is present in the surface coating composition in an amount within the range of 5-8 parts by weight of filler to 1 part by weight of polyvinyl alcohol.

12. A planographic printing plate as claimed in claim 8 in which the pigment comprises clay.

13. A planographic printing plate as claimed in claim 12 in which the clay is present in the base coating composition in an amount within the range of l-4 parts by weight per 1 part by weight of polyvinyl alcohol.

14. A planographic printing plate as claimed in claim 8 in which the polyvalent metal salts are present in the base coat in an amount within the range of 2-1 5 percent by weight.

15. A planographic printing plate as claimed in claim 8 in which the polyvalent metal salts are a mixture of bivalent metal salts and trivalent metal consisting of the ratio of l-4 parts by weight of bivalent metal salts to 1 part by weight of trivalent metal salts.

16. A planographic printing plate as claimed in claim 15 in which the bivalent metal salts are salts in which the anion is selected from the group consisting of a chloride, nitrate, for-' mate, acetate and propionate and in which the cation is selected from the group consisting of zinc, calcium, nickel, copper, cobalt, barium and strontium and in which the trivalent metal salt is selected from the group consisting of the chloride, nitrate, formate and acetate of aluminum.

lfi planographic printing plate as claimed in claim 8 in which the melamine formaldehyde resin is present in the base coat in an amount within the range of 0.4-4 percent by weight.

16. A planographic printing plate as claimed in claim 8 in which the ethyl acrylate-acrylonitrile-methacrylic acid resin is present in the base coat in an amount within the range of 3-15 percent by weight.

19. A planographic printing plate as claimed in claim 8 in which the lithographic surface coating contains a melamine formaldehyde resin in an amount within the range of 0.5-5 percent by weight.

20. A planographic printing plate as claimed in claim 8 in which clay is present in the lithographic surface coating in an amount within the range of 50-60 percent by weight and colloidal silica is present within the range of 5-40 percent by weight.

* t l l l 

2. A planographic printing plate as claimed in claim 1 in which the hydrophilic colloid adhesive of the lithographic surface coating is selected from the group consisting of polyvinyl alcohol and hydroxyethylated starch.
 3. A planographic printing plate as claimed in claim 1 in which the hydrophilic colloid adhesive in the base coat is selected from the group consisting of polyvinyl alcohol and hydroxyethylated starch.
 4. A planographic printing plate as claimed in claim 1 in which the pigment in the lithographic surface comprises colloidal silica containing substituted aluminum.
 5. A planographic printing plate as claimed in claim 1 in which the soluble phosphate is selected from the group consisting of monosodium phosphate, disodium monohydrogen phosphate, tetrasodium pyrophosphate, sodium acid pyrophosphate, sodium tripolyphosphate, sodium hexametaphosphate and the corresponding phosphates in which ammonium or potassium are substituted for sodium.
 6. A planographic printing plate as claimed in claim 1 in which the adhesive of the base coat includes an amide aldehyde resin.
 7. A planographic printing plate as claimed in claim 1 in which the adhesive of the base coat includes an amide aldehyde resin and an ethyl acrylate-acrylonitrile-methacrylic acid resin.
 8. A planographic printing plate comprising a base sheet and the combination on the surface of the base sheet of an outermost lithographic surface coating and a base coating immediately underlying the surface coating, in which the surface coating is formed of the dried residue of an aqueous composition the solids of which consist essentially of polyvinyl alcohol, a soluble phosphate and a filler or pigment without polyvalent metal salts and in which the base coat is formed of the dried residue of an aqueouS composition in which the solids consist essentially of polyvinyl alcohol, a melamine formaldehyde resin, an ethyl acrylate-acrylonitrile-methacrylic acid resin, filler or pigment and water soluble polyvalent metal salts.
 9. A planographic printing plate as claimed in claim 8 in which the filler or pigment comprises the combination of clay and a colloidal silica.
 10. A planographic printing plate as claimed in claim 8 in which the filler or pigment includes a colloidal silica in which at least a part of the silicon is replaced with aluminum.
 11. A planographic printing plate as claimed in claim 8 in which the filler is present in the surface coating composition in an amount within the range of 5-8 parts by weight of filler to 1 part by weight of polyvinyl alcohol.
 12. A planographic printing plate as claimed in claim 8 in which the pigment comprises clay.
 13. A planographic printing plate as claimed in claim 12 in which the clay is present in the base coating composition in an amount within the range of 1-4 parts by weight per 1 part by weight of polyvinyl alcohol.
 14. A planographic printing plate as claimed in claim 8 in which the polyvalent metal salts are present in the base coat in an amount within the range of 2-15 percent by weight.
 15. A planographic printing plate as claimed in claim 8 in which the polyvalent metal salts are a mixture of bivalent metal salts and trivalent metal salts in the ratio of 1-4 parts by weight of bivalent metal salts to 1 part by weight of trivalent metal salts.
 16. A planographic printing plate as claimed in claim 15 in which the bivalent metal salts are salts in which the anion is selected from the group consisting of a chloride, nitrate, formate, acetate and propionate and in which the cation is selected from the group consisting of zinc, calcium, nickel, copper, cobalt, barium and strontium and in which the trivalent metal salt is selected from the group consisting of the chloride, nitrate, formate and acetate of aluminum.
 17. A planographic printing plate as claimed in claim 8 in which the melamine formaldehyde resin is present in the base coat in an amount within the range of 0.4-4 percent by weight.
 18. A planographic printing plate as claimed in claim 8 in which the ethyl acrylate-acrylonitrile-methacrylic acid resin is present in the base coat in an amount within the range of 3-15 percent by weight.
 19. A planographic printing plate as claimed in claim 8 in which the lithographic surface coating contains a melamine formaldehyde resin in an amount within the range of 0.5-5 percent by weight.
 20. A planographic printing plate as claimed in claim 8 in which clay is present in the lithographic surface coating in an amount within the range of 50-60 percent by weight and colloidal silica is present within the range of 5-40 percent by weight. 